Conserving Bats and Buildings

A Natural Synergy

Kelly Gunnell

A pipistrelle bat in flight (Photo: Hugh Clark)

Long associated in popular culture
with horror and the supernatural, bats
are in desperate need of a PR makeover.
Contrary to popular opinion, bats are
not mice with wings. They are, in fact, more
closely related to humans than to rodents.
These socially complex mammals usually have
just one young per year and look after their
offspring fastidiously, often forming crèches
that allow the mothers to go out to forage
while other females look after the pups.

Bats have long life-spans for their size.
The record is 40 years for a ringed Brandt’s
bat. They do not make nests and they do not
cause structural damage to buildings. Bats
are not blind, they can see reasonably well
but have evolved echolocation to help them
with the difficult task of hunting tiny insects
in the dark. Bats are an important part of
the UK’s biodiversity, the 17 species of bat
account for nearly one third of our national
mammal species (globally there are over
1,000 bat species). All bats in the UK eat
insects – a single pipistrelle bat can eat up to
3,000 midges in a single night. Bats are thus
an important part of the ecosystem and their
presence is an indication of a healthy and
biodiverse landscape.

One stereotype does hold true, however:
bats do like to roost in old buildings. In fact,
bats and people have been sharing dwellings
for thousands of years, making these unique
creatures part of our cultural heritage as well
as part of our natural heritage. Consequently,
when conserving old buildings, there is a legal
and moral imperative to preserve bat roosts.

WHY DO BATS ROOST IN BUILDINGS?

CASE STUDY 1: THE NEED FOR SURVEYING

(Photo: Pat Waring/Ecology Services UK Ltd)

A large aisled barn in East
Lancashire, dating from 1605,
was undergoing restoration.
The roof structure was in a
poor condition and needed
substantial repairs and
refurbishment. The building
comprised coursed sandstone
rubble with quoins and a
stone-slate roof. Bat roosting
features included wall cavities
and a very large number of
mortice joints in the rafters,
aisle posts and kingpost roof
supports. Bat species found
in the barn included common
and soprano pipistrelle,
brown long-eared and Natterer’s Bats. As part of the preparations for roof repairs, surveys
were undertaken throughout the year in 2009, using a combination of building searches,
remote detectors (every month) and manned surveys (May to September). Bats
were found to be active in all months of the year and the building supported roosting bats in
every month. The information gained will be an essential part of the mitigation strategy.

About half of the bat species in the world use
holes in trees for roosting. The other half use
either caves or cavities. However, as ‘natural’
roosting sites have become scarce due to
development and changing land-use, bats
have adapted to use buildings, like barn owls
or swifts, and are a building-reliant species.

Buildings offer a range of possible roosting
opportunities that mimic those found in the
natural environment and are often cleaner,
safer and warmer spaces than natural roosting
sites. Crevice-like or tree cavity type spaces
include those found in both modern and
traditional houses such as behind fascia and
barge boarding, spaces beneath roof tiles, wall
coatings, hollow mortice joints, rain gutters
and chimneys. Cave-like spaces include attics and cellars that are dark, with stable
temperatures and humidity. These spaces may
be found in a range of structures including
farm buildings, historic houses, castles,
churches and terraced houses.

Bats are much smaller than people
tend to think; some species only need a
15-20mm gap to access a roosting space.
Most bat species cluster together tightly to
generate heat and, given that it’s possible
to squeeze about 200 pipistrelle bats into
a space the size of a shoe box, you can
imagine how difficult it can be to find bats
roosting in the dark corners of a building.

Bats do not make nests or chew electrical
cables. The most obvious sign of their presence
is bat droppings but even these can be hard
to find, especially in a loft. Bat droppings are
frequently mistaken for mouse droppings but
can be identified by the ‘crumble test’. Bat
droppings consist largely of insect remains
and crumble easily to a powder of semi-shiny
fragments. Rodent droppings are smooth and
plastic, quickly becoming hard. They cannot
be crumbled. The droppings of UK bats do
not present any known health hazards; they
can be swept up safely and make an excellent
garden fertiliser. Large accumulations may
reflect use of the same roost over a number of
years rather than large numbers of bats at any
one time.

Other signs to look for are grease
marks on the rafters, urine splashes, cobweb-free
corners, or insect remains from a feeding
perch. If in doubt, contact your local bat group
or a professional ecologist to conduct a survey.

Unfortunately for bats, even man-made
roosts are now under threat. Demolition
of old buildings, renovation, change of use,
artificial lighting and the move towards airtight
buildings, all have implications for the
bat populations that use buildings. Combined
with the loss of foraging habitat, this has
meant that bat populations have suffered
drastic losses in the last century. Bats are
therefore afforded very strict legal protection
which must be considered when carrying out
any building work.

SURVEYS, PLANNERS AND THE LAW

CASE STUDY 2: MITIGATION WITH A BAT BARN

(Photo: Pat Waring/Ecology Services UK Ltd)

A former textile mill, built in 1888 in a river
valley in West Yorkshire, was proposed for
development into a series of apartments.
The surrounding landscape is ideal for bats
and includes at least six species. Within
the mill buildings, common and soprano
pipistrelle bats have been recorded during
spring and summer months, with at least
one of the species hibernating in ground
floor door lintels. The buildings also
supported nesting swifts. As part of the
preparation for development a dedicated
bat barn was constructed in 2008 within
100m of the mill, adjacent to woodland and
the river. Subsequent monitoring found bat
droppings in the bat barn in 2010.

European and domestic legislation dictates
that any structures or places which bats
use for shelter are protected from damage
or destruction, whether occupied by
bats or not. This legislation means that
planning authorities have an obligation
to consider whether bats are likely to be
affected by a proposed development. An
application cannot be granted planning
permission unless it includes all the
information needed to make sure that bats
will not be affected by building works.

It is for this reason that planning
authorities will often ask for a bat survey to be
carried out and the results submitted with the
initial planning application. The survey will
need to be undertaken by a licensed ecologist,
who will inspect the site and building
internally and externally to determine the
likelihood of bats being present.

If the potential for the site to support bats
is considered high, additional activity surveys
will be carried out at dawn or dusk. These surveys aim to confirm the species present,
the type of roost and identify any access
points and commuting routes. Once this
information has been collected, an assessment
is made as to whether the planned works
will impact the bats. If necessary, measures
will then be suggested to reduce the impact
to an acceptable level. In some instances
where the impact is considered significant, an
application for a European Protected Species
(EPS) licence will be needed. The EPS licence
will outline conditions to mitigate for the
unavoidable impacts. This does not stop the
work from going ahead, but tries to ensure
that any work doesn’t negatively impact the
bats and is within the law.

The licensing process can take up to
six weeks so it is always recommended
that an ecologist is contacted as early
on in the process as possible. This
allows for any alterations to the scope
and timing of works to be incorporated
into schedules and prevents delays.

STEPS TO TAKE IF BATS ARE DISCOVERED

When undertaking building or development
work where bats may be present, the following
steps should be taken:

contract an ecological consultant for
guidance throughout the project

undertake a bat survey at the
appropriate time of year

if bats are present, compile a mitigation/
compensation plan or method statement
for everyone involved including
architects and building contractors

incorporate the bat survey report and
mitigation plan/method statement
into the planning application

apply for planning permission

if necessary, apply for an EPS licence
(the planning permission will be needed
as part of the licence application)

if a licence is granted, carry out works
with an ecologist’s supervision

carry out a compliance check to ensure that
mitigation is being properly implemented

monitor the site after the completion
of the mitigation to gauge the
response of the bat population.

BAT ROOST MITIGATION

CASE STUDY 3: MITIGATION IN A LISTED BUILDING

(Photo: CTT Sustainable Architect)

Clarkencombe Lodge is a
Grade II* listed gatehouse and part of the Ashton
Court Estate in Bristol.
Plans to refurbish the lodge as a
residential space had
to allow for the five species
of bat found on the site to
continue to use the building
and to enhance the maternity
roost. The bat species
included greater and
lesser horseshoe bats, brown
long-eared, common pipistrelle and barbastelle bats.
Mitigation involved isolating the living accommodation from the designated bat areas,
including the roof void. One tower of the gatehouse was also allocated for the bats,
creating a route from the roof to the ground level single storey modern extension, which
was modified to provide roosting ledges and a maternity environment. All windows and doors
were removed from the extension and the building covered with planting. Its interior layout was
designed by a bat consultant. Bat entrances and exits were created in the roof and tower.
Canopy cover from the building to the adjacent woodland was ensured. A camera installed
for ongoing monitoring has shown an increase in the use of the building by bats
and that the improved maternity roost is in use.

The main aim of bat roost mitigation is to
allow for the conservation status of bats to
be maintained or enhanced during and after
the development.

Bat roost mitigation comes
in many forms. In most cases, mitigating for
bats during building works involves changing
the construction schedule to avoid sensitive
breeding seasons and changing the scope of
the work to maintain the bat entrances. Bat
access points are created or maintained by
making gaps either using existing materials
or specially designed bat access tiles or bricks
that allow bats to re-enter the roost once the
work is finished.

More complicated forms of mitigation
involve replacing roosts within the building
or creating purpose-built roosts such as bat
houses or bat barns. Maintaining roosts in
situ is always the preferred option. However,
in some cases purpose-built bat houses may
be considered, as long as the risks of non-adoption
by bats are minimised through
careful design and site selection. In all
cases, bat mitigation should aim to mimic
or optimise the original roosting conditions,
particularly thermal properties and access to
favourable landscape features.

Below are some design principles for
ex situ roost conservation:

situate the replacement roost as close
as possible to the roost to be lost

match the replacement roost closely
in terms of size, height and aspect

situate the replacement roost close to
existing flight-lines and have an entrance
close to appropriate habitat to maximise
chances of the bats finding and adopting it

design the new roost to provide a suitable
thermal regime for the target bat species

provide a variety of roosting
opportunities and thermal regimes

make the building resistant to vandalism

arrange for the long-term integrity and
security of the replacement roost.

ROOST WEBSITE

The frustration for homeowners and ecologists
alike is that even the best thought-out and
designed roost mitigation may not succeed.

Bats can be fickle and our understanding of
them is incomplete. Each species has its own
unique preferences for temperature, roost
size, access points and proximity to suitable
landscape features and vegetation. These
requirements can also change regionally and
seasonally. Determining all these criteria
and putting them together for a successful
mitigation strategy is immensely challenging.

Combine this with a lack of systematic follow-up
and monitoring and you can see why the
realm of bat roost mitigation has remained
more of an art than a science. The licensing
procedure has made it difficult for builders,
ecological consultants, architects and
homeowners to keep track of the mitigation
measures used in the past and whether they
were successful.

New bat access in one of the turrets of Clarkencombe Lodge, Bristol
(Photo: CTT Sustainable Architect)

To address this, the Bat Conservation
Trust has developed a new web resource
called Roost which enables users to upload
information about roosts, mitigation strategies
and explore case studies. Users can browse
case studies using a number of criteria, make
comments and access information on general
design principles and resources for bat roost
mitigation. The aim is that Roost will become
a knowledge hub for anyone undertaking or
involved in bat roost mitigation. In the long term, the collected information could provide
a wealth of insight into what it takes to make a
successful and attractive replacement roost.

It is hoped that by sharing information
about effective mitigation, we can find new
ways to enable bats to live happily alongside us
for centuries to come.

Author

KELLY GUNNELL is the built environment
officer for the Bat Conservation Trust. The
Built Environment Project aims to raise
awareness and facilitate solutions for
bat conservation in the construction and
building sector.